1. Field of the Invention
The present invention related to a device for presuming the pressure level of a pressure accumulator prepared for accumulating a fluid under a compression of an elastic pressurization medium to selective supply the fluid therefrom with a timely supplement of the fluid thereto by pump means.
2. Description of the Prior Art
The pressure accumulators of the above-mentioned type are now popular in the art of the automatic behavior control of automobiles, serving as a source of a pressurized brake fluid for automatically actuating wheel cylinders of the automobile, so as to apply a controlled braking to each of the wheels of the automobile for various behavior controls thereof in such a manner that a front wheel at the outside of the turn is braked when the automobile is suppressed from getting into a spin, while a rear wheel or wheels, particularly a rear wheel at the inside of a turn is braked when the automobile is suppressed from getting to a driftout. In order to apply such a controlled braking to each of the wheels by an automobile behavior control device, the pressure level of the accumulator serving as the pressure source of the brake fluid is one of the essential parameters to be known.
The working pressure level of those accumulators is generally around 15 MP (mega pascal). Although various types of pressure sensors are available for use with detecting pressure of such a level, those sensors are much more expensive than the pressure switches which may only detect if a certain predetermined pressure level was traversed or not.
It is described in Japanese Patent Laid-open Publication 8-230648 to operate a pressure accumulator of a hydraulic brake circuit means of an automobile by presuming the pressure level thereof based upon a pre-estimated pressure increase gradient of the accumulator under a pumping of the brake fluid thereto by certain pump means therefor.
However, the pressure increase gradient of a combination of a certain accumulator and certain pump means will widely change according to an allowance for performances of the products, a fitting of performances according to the period of use of the products, particularly in an early stage of use, the ambient temperature according to the seasons which substantially affects the viscosity of the fluid, and so on. Therefore, it is anticipated that the pre-setting of the pressure increase gradient will not necessarily guarantee an expected control of the accumulators in the actual operation.
On the other hand, in the case of the brake fluid accumulators incorporated in the automatic brake hydraulic circuits of automobiles, it is important that the fluid pressure of the accumulator generally maintained around 15 Mp as describe above does not lower so much as to be below generally 10 Mp during the operation of the automatic brake hydraulic circuit, in order to ensure the sound operation of the automatic behavior control system of the automobile. Therefore, it is generally essential for the automatic brake hydraulic circuit that at least one pressure switch is provided for detecting that the fluid pressure of the accumulator does not lower below a limit such as 10 Mp.
Further, with respect to those automatic brake hydraulic circuits of automobiles adapted to work at such a relatively high pressure as around 15 Mp, there is a special situation that, although the pressure of the brake hydraulic circuit held in communication with the accumulator may be maintained at such a high operating pressure level during a short rest of the automobile up to a few hours often met in daily use of the automobiles, when the automobile is left in a continuous rest over a night, it is desirable that the high operating pressure in the circuit including the accumulator is releases to ensure a longer life of the brake hydraulic circuit. Therefore, indeed, such brake hydraulic circuits of automobiles are generally designed to have a leak of the brake fluid, so as to lower the pressure of the circuits to be substantially below the normal working pressure after the lapse of a certain time when the pump means are not operated in the meantime, thus requiring a substantial new charge of the accumulator when the automobile is restarted after such a continuous rest.